This invention relates generally to a method and apparatus for printing monochromatic imaging onto photosensitive media by spatially and temporally modulating a light beam, and more particularly to a film recording apparatus that allows selection of a light source of a preferred wavelength from among a set of available light sources having different wavelengths.
Conventional printers generally adapted to record images provided from digital data onto photosensitive media apply light exposure energy that may originate from a number of different sources and may be modulated in a number of different ways. In photoprocessing apparatus, for example, light exposure energy can be applied from a CRT-based printer. In a CRT-based printer, the digital data is used to modulate a Cathode Ray Tube (CRT) which provides exposure energy by scanning an electron beam of variable intensity along its phosphorescent screen. Alternately, light exposure energy can be applied from a laser-based printer, as is disclosed in U.S. Pat. No. 4,728,965 (Kessler, et al.) In a laser-based printer, the digital data is used to modulate the duration of laser on-time or intensity as the beam is scanned by a rotating polygon onto the imaging plane.
CRT- and laser-based printers perform satisfactorily for photoprocessing applications, that is, for printing of photographs for consumer and commercial markets. However, in an effort to reduce cost and complexity, alternative technologies have been considered for use in photoprocessing printers. Among suitable candidate technologies under development are two-dimensional spatial light modulators.
Two-dimensional spatial light modulators, such as those using a digital micromirror device (DMD) from Texas Instruments, Dallas, Tex., or using a liquid crystal device (LCD) can be used to modulate an incoming optical beam for imaging. A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate incident light from a light source by modulating the polarization state of the light. Polarization considerations are, therefore, important in the overall design of support optics for a spatial light modulator.
There are two basic types of spatial light modulators in current use. The first type developed was the transmissive spatial light modulator, which, as its name implies, operates by modulating an optical beam that is transmitted through individual array elements. The second type, a later development, is a reflective spatial light modulator. As its name implies, the reflective spatial light modulator operates by modulating a reflected optical beam through individual array elements. A suitable example of an LCD reflective spatial light modulator relevant to this application utilizes an integrated CMOS backplane, allowing a small footprint and improved uniformity characteristics.
Conventionally, LCD spatial light modulators have been developed and employed for digital projection systems for image display, such as is disclosed in U.S. Pat. No. 5,325,137 (Konno et al.) and in miniaturized image display apparatus suitable for mounting within a helmet or supported by eyeglasses, as is disclosed in U.S. Pat. No. 5,808,800 (Handschy et al.) LCD projector and display designs in use typically employ one or more spatial light modulators, such as using one for each of the primary colors, as is disclosed in U.S. Pat. No. 5,743,610 (Yajima et al.).
It is instructive to note that imaging requirements for projector and display use (as is typified in U.S. Pat. Nos. 5,325,137; 5,808,800; and 5,743,610) differ significantly from imaging requirements for printing. Projectors are optimized to provide maximum luminous flux to a screen, with secondary emphasis placed on characteristics important in printing, such as contrast and resolution. Optical systems for projector and display applications are designed for the response of the human eye, which, when viewing a display, is relatively insensitive to image artifacts and aberrations and to image non-uniformity, since the displayed image is continually refreshed and is viewed from a distance. However, when viewing printed output from a high-resolution printing system, the human eye is not nearly as xe2x80x9cforgivingxe2x80x9d to artifacts and aberrations and to non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. For this reason, there can be considerable complexity in optical systems for providing a uniform exposure energy for printing. Even more significant are differences in resolution requirements. Adapted for the human eye, projection and display systems are optimized for viewing at typical resolutions such as 72 dpi or less, for example. Photographic printing apparatus, on the other hand, must achieve much higher resolution, particularly apparatus designed for micrographics applications, which can be expected to provide 8,000 dpi for some systems. Thus, while LCD spatial light modulators can be used in a range of imaging applications from projection and display to high-resolution printing, the requirements on supporting optics can vary significantly.
Largely because spatial light modulators can offer significant advantages in cost and size, these devices have been proposed for different printing systems, from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748 (Sarraf), to area printing systems such as the system described in U.S. Pat. No. 5,652,661 (Gallipeau et al.) One approach, using a Texas Instruments DMD as shown in U.S. Pat. No. 5,461,411 offers advantages common to spatial light modulator printing such as longer exposure times using light emitting diodes as a source as shown in U.S. Pat. No. 5,504,514. However, DMD technology is very specific and not widely available. As a result, DMDs may be expensive and not easily scaleable to higher resolution requirements. The currently available resolution using DMDs is not sufficient for all printing needs. Furthermore, there is no clear technology path to increased resolution with DMDs.
A preferred approach for photoprocessing printers uses an LCD-based spatial light modulator. Liquid crystal modulators can be a low cost solution for applications requiring spatial light modulators. Photographic printers using commonly available LCD technology are disclosed in U.S. Pat. Nos. 5,652,661; 5,701,185 (Reiss et al.); and U.S. Pat. No. 5,745,156 (Federico et al.) Although the present application primarily addresses use of LCD spatial light modulators, references to LCD in the subsequent description can be generalized, for the most part, to other types of spatial light modulators, such as the DMD noted above.
Primarily because of their early development for and association with screen projection of digital images, spatial light modulators have largely been adapted for continuous tone (contone) color imaging applications. Unlike other digital printing devices, such as the CRT and laser-based devices mentioned above that scan a beam in a two-dimensional pattern, spatial light modulators image one complete frame at a time. Using an LCD, the total exposure duration and overall exposure energy supplied for a frame can be varied as necessary in order to achieve the desired image density and to control media reciprocity characteristics. Advantageously, for photoprocessing applications, the capability for timing and intensity control of each individual pixel allows an LCD printer to provide grayscale imaging.
Most printer designs using LCD technology employ the LCD as a transmissive spatial light modulator, such as is disclosed in U.S. Pat. Nos. 5,652,661 and 5,701,185. However, the improved size and performance characteristics of reflective LCD arrays have made this technology a desirable alternative for conventional color photographic printing, as is disclosed in commonly assigned, copending U.S. patent application Ser. No. 09/197,328, filed Nov. 19, 1998, entitled xe2x80x9cReflective Liquid Crystal Modulator Based Printing Systemxe2x80x9d by Ramanujan et al. As is described in the Ramanujan application, color photographic printing requires multiple color light sources applied in sequential fashion. The supporting illumination optics are required to handle broadband light sources, including use of a broadband beamsplitter cube. The optics system for such a printer must provide telecentric illumination for color printing applications. In summary, in the evolution of photoprocessing systems for film printing, as outlined above, it can be seen that the contone imaging requirements for color imaging are suitably met by employing LCD spatial light modulators as a solution.
Printing systems for micrographics or Computer-Output-Microfilm (COM) imaging, diagnostic imaging, and other specialized monochrome imaging applications present a number of unique challenges for optical systems. In the COM environment, images are archived for long-term storage and retrievability. Unlike conventional color photographic images, microfilm archives, for example, are intended to last for hundreds years in some environments. This archival requirement has, in turn, driven a number of related requirements for image quality. For image reproduction quality, for example, one of the key expectations for micrographics applications is that all images stored on archival media will be written as high-contrast black and white images. Color film is not used as a medium for COM applications since it degrades much too quickly for archive purposes and is not capable of providing the needed resolution. Grayscale representation, meanwhile, has not been available for conventional micrographics printers. Certainly, bitonal representation is appropriate for storage of alphanumeric characters and for standard types of line drawings such as those used in engineering and utilities environments, for example. In order to record bitonal images onto photosensitive media, exposure energy applied by the printer is either on or off, to create high-contrast images without intermediate levels or grayscale representation.
In addition to the requirement for superb contrast is the requirement for high resolution of COM output. COM images, for example, are routinely printed onto media at reductions of 40xc3x97 or more. Overall, micrographics media is designed to provide much higher resolution than conventional dye-based media provides for color photographic imaging. To provide high resolution, micrographics media employs a much smaller AgX grain size in its photosensitive emulsion. Optics components for COM systems are correspondingly designed to maximize resolution, more so than with optical components designed for conventional color photoprocessing apparatus.
Conventional COM printers have utilized both CRT-and laser-based imaging optics with some success. However, there is room for improvement. For example, CRT-based printers for COM use, such as disclosed in U.S. Pat. No. 4,624,558 (Johnson) are relatively costly and can be bulky. Laser-based printers, such as disclosed in U.S. Pat. No. 4,777,514 (Theer et al.) present size and cost constraints and can be mechanically more complex, since the laser imaging system with its spinning polygon and beam-shaping optics must be designed specifically for the printer application. In addition, laser printers exhibit high-intensity reciprocity failure when used with conventional photosensitive media, thus necessitating the design of special media for COM use.
More recent technologies employed for COM imaging include use of linear arrays such as linear light-emitting diode (LED) arrays, for example, as are used in the Model 4800 Document Archive Writer, manufactured by Eastman Kodak Company, Rochester, N.Y. Another alternative is use of a linear light-valve array, such as is disclosed in U.S. Pat. No. 5,030,970 (Rau et al.) However, with exposure printheads using linear arrays, COM writers continue to be relatively expensive, largely due to the cost of support components and to the complexity of drive electronics. There is a long-felt need to lower cost and reduce size and complexity for COM devices, without sacrificing performance or robustness.
A well-known shortcoming of conventional COM printers relates to the interdependence between COM printer design and the exposure sensitivity characteristics of a specific photosensitive media type. Currently, a particular type of COM printer is designed to write only on a single type of COM media. Conversely, a single type of COM media can only be used in a particular type of COM printer. The exposure optics of a particular type of COM printer are designed to apply specific levels of exposure energy over a specific range of wavelengths to the COM media. Because of this constraint, a customer who purchases a COM printer of specific manufacture and model type can use that COM printer only with COM media that has been developed specifically for that printer, or with a very limited number of other types of media having similar characteristics. This is true even though the same media handling subsystem used in the COM printer could be capable of routing different types of photosensitive media from a film supply, through an exposure section, and to a film processing or film storage unit for exposed media.
Exposure wavelength is one important characteristic that constrains COM printer use to a specific media. Existing COM printers use monochromatic light as the source of exposure energy. Different COM media are designed for optimum performance with monochromatic exposure light at different wavelengths. For example, the KODAK Archive Storage Media 3459 is optimized for exposure wavelengths near 685 nm. KODAK IMAGELINK DL Microfilm, on the other hand, is designed for optimal sensitivity when exposed at 633 nm.
This interdependence of COM printer and COM media characteristics is disadvantageous from a number of perspectives. Development of an improved COM printer can be constrained by the requirement that exposure optics provide only a specific output wavelength. Development and marketing of an improved COM film can be constrained either by the requirement that the COM film be used at exposure wavelengths available with existing COM printers or by the requirement that a new COM printer be developed, in order to provide exposure energy at the proper wavelength. These constraints add cost to the production of both COM media and COM printing apparatus and limit the flexibility of COM customers to use a preferred printer or media type for a given situation.
Conventional COM printing apparatus can be adjusted somewhat for slight media sensitivity variation, but such routine adjustments are made only in order to adapt to anticipated batch-to-batch media variability over a narrow range. For this purpose, Calibration Look-Up Tables (LUTs) are used with some systems to adjust exposure characteristics (exposure time and intensity) to compensate for slight drift (such as might be due to media aging) or batch sensitivity differences. However, this type of solution would not be suitable for handling different media types having different wavelength sensitivity. Even though intensity and timing of exposure energy can be adjusted, these exposure factors cannot adequately compensate for media wavelength sensitivity differences over more than a narrow range without having an objectionable impact on image quality.
Conventional exposure optics systems are limited to the use of a single type of exposure light source. Depending on the type of light source used, it can be possible to provide exposure light at different wavelengths. For example, where the exposure light source is a halogen bulb, it would be possible to provide interchangeable filters arranged to allow selection from among multiple exposure wavelengths, depending on the choice of filter. However, such a solution would require manual insertion of a filter element or, if automated, moving parts for positioning a filter in the light path. It could also be possible to provide multiple lasers, for example, and allow an operator-initiated or automated selection of a specific laser in the exposure optics path for a particular COM media. However, such a solution requires expensive components and would not allow compact packaging without introducing a significant amount of mechanical complexity. Any practical solution for providing a selectable exposure wavelength must meet the goals of low-cost, compact packaging, and mechanical simplicity that would not be provided by conventional COM light sources. Furthermore, where possible, automated mechanisms would be preferred over manual methods for adapting a COM printer to a specific COM media.
Thus, it can be seen that there is a need for an improved COM printing apparatus that is inexpensive, compact, and robust, that allows the use of alternate types of COM media where the COM media have different exposure characteristics and that allows automated sensing and response to the type of COM media loaded.
It is an object of the present invention to provide a printing apparatus using a spatial light modulator for imaging onto photosensitive media, wherein the printing apparatus is capable of using any one of a number of possible monochromatic light sources.
According to one aspect of the present invention an apparatus prints monochrome images from digital image data onto a selected photosensitive medium that is selected from a plurality of photosensitive media compatible with the monochrome printing apparatus. A light source, which is selectable, selects from a plurality of light source elements a monochromatic light source that is suited to the selected photosensitive medium. A uniformizer uniformizes the light that is emitted from the monochromatic light source. A polarizer for filtering the uniformize light provides a polarized beam having a predetermined polarization state. A spatial light modulator has a plurality of individual elements capable of altering the polarization state of the polarized beam to provide an exposure beam for printing, the state of the elements controlled according to the digital image data. A first lens assembly directs the polarized beam to the spatial light modulator and a second lens assembly directs the exposure beam onto the selected photosensitive medium.
According to one embodiment of the present invention, any one of a set of monochromatic light source elements can be selectively energized as the light source for exposing the photosensitive media. The monochromatic exposure light is passed through a uniformizer or integrator to provide a source of spatially uniform, monochromatic light for the printing apparatus. The monochromatic light is then polarized and passed through a beamsplitter, which directs a polarized beam onto a spatial light modulator. Individual array elements of the spatial light modulator, controlled according to digital image data, are turned on or off in order to modulate the polarization rotation of the incident light. Modulation for each pixel can be effected by controlling the level of the light from the light source, by control of the drive voltage to each individual pixel in the spatial light modulator, or by controlling the duration of on-time for each individual array element. The resulting light is then directed through a lens assembly to expose the photosensitive medium.
According to a preferred embodiment of the present invention, the plurality of monochromatic light sources is made available by the use of an array of LEDs, wherein different groupings of LEDs within the array can be selectively energized to provide optical exposure energy at different wavelengths.
An advantage of the present invention is that it allows a single monochrome printing apparatus to be able to use a range of media types, where the media types differ in sensitivity to exposure wavelength. This allows an existing printing apparatus to take advantage of new media types as well as improvements in media performance. Conversely, this allows a new printing apparatus to be designed to use both newly introduced and existing media types.
A further advantage of the present invention is that it allows the development of lower cost photosensitive media by allowing variability over the range of exposure wavelengths used for imaging.
A further advantage of the present invention is that it can provide wavelength selectivity without introducing any moving part and without appreciably increasing the cost, size, or mechanical complexity of the printer.
A further advantage of the present invention is that it provides a mechanism for automatically selecting an appropriate light source, based on detecting the type of media loaded in the printing apparatus, thus eliminating operator interaction and possible operator error.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there are shown and described illustrative embodiments of the invention.